Dr McBeefington
Restricted
This is a difficult topic to discuss because as newer star wars stuff comes out, the more ridiculous and unbelievable it becomes, even in the realm of Star Wars.
Let's take a look:
The Order of the Sith Lords were hidden for a thousand years and concentrated their power between two people. Because there were only numbering two, they had to share sith knowledge between the both of them, as opposed to the thousands of other sith. One may say that complete domination for 20 years after preparation for 1000 years is a pretty terrible ratio. On the other hand, they successfully wiped out the Jedi Order and watched the Jedi slowly create a new one, knowing it might take centuries to achieve full strength again. On the OTHER hand, it's unclear if the Sith would have ever conquered the Republic had Sidious not been born. We have no idea as to what extent the Sith corrupted the Republic for a millennium..
The Order of One is just a perversion of the ancient sith order, except there is no sith/dark council. They piggybacked on the reorganized Empire to take control of the galaxy after barely 100 years in existence. In terms of efficiency, I think they win. However, it doesn't appear that they possess anywhere near the level of dominance shared by Sidious and his Empire. They're constantly fighting Jedi and amongst themselves, something not seen between one master and one apprentice. Also, some of the sith, Wyyrlok and Krayt in particular, seem to have extensive knowledge of arcane force techniques. They may be the only ones that rival some of the sith lords in the Order of Two..
Anyways, that's all I can think up on the fly. They both have advantages and disadvantages but I think the Order of the Sith Lords win by a hair.